Berlin 2012 – scientific programme
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DS: Fachverband Dünne Schichten
DS 34: Organic thin films II: Interface spectroscopy
DS 34.5: Talk
Thursday, March 29, 2012, 16:00–16:15, H 2032
Influence of interface dipoles on the open circuit voltage of polymer/PCBM based photovoltaic cells — •Johannes Frisch1, Stefanie Winkler1, Jens Niederhausen1, Patrick Amsalem1, Antje Vollmer2, Marcel Schubert3, Dieter Neher3, Eduard Preis4, Ullrich Scherf4, and Norbert Koch1,2 — 1Humboldt-Universität zu Berlin, Germany — 2Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Germany — 3Universität Potsdam, Germany — 4Bergische Universität Wuppertal, Germany
In heterojunction organic photovoltaic cells (OPVCs) the offset between the highest occupied molecular level of the donor and the lowest unoccupied molecular level of the acceptor (i.e., the photovoltaic gap) sets an upper limit for the open circuit voltage (Voc). Commonly, vacuum level alignment (VLA) at the donor/acceptor interface is assumed. In contrast, we found interface dipoles (ID) at such interfaces that affect the photovoltaic gap and therefore Voc. We report ultraviolet photoelectron spectroscopy measurements for three different polymer-PCBM heterojunctions, comprising the donors P3HT, poly(9,9'dioctylfluorene-alt-4,7-bis(2,5-thiendiyl)-2,1,3-benzothiadiazole) (PFTBTT) and poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']dithiophene)-alt-4,7(2,1,3-benzothiadiazole)] (PCPDTBT). The energy level alignment found experimentally for these heterojunctions are discussed in relation to the Voc achieved in corresponding OPVCs. In contrast to assumed VLA, the photovoltaic gap for the P3HT/PCBM junction is 0.15 eV wider due to the ID, and for PFTBTT/PCBM the photovoltaic gap is narrower by 0.15 eV.